Production method for ion exchange membrane for alkali chloride electrolysis, and production method for alkali chloride electrolysis apparatus

ABSTRACT

To provide a production method whereby an ion exchange membrane for alkali chloride electrolysis can be obtained which has high current efficiency, little variation in current efficiency and high alkaline resistance. This is a method for producing an ion exchange membrane  1  having a layer (C)  12  containing a fluorinated polymer (A) having carboxylic acid type functional groups, by immersing an ion exchange membrane precursor film having a precursor layer (C′) containing a fluorinated polymer (A′) having groups convertible to carboxylic acid type functional groups, in an aqueous alkaline solution comprising an alkali metal hydroxide, a water-soluble organic solvent and water, and converting the groups convertible to carboxylic acid type functional groups to carboxylic acid functional groups, wherein the concentration of the water-soluble organic solvent is from 1 to 60 mass % in the aqueous alkaline solution (100 mass %); the temperature of the aqueous alkaline solution is at least 40° C. and less than 80° C.; and the proportion of structural units having carboxylic acid type functional groups in the fluorinated polymer (A) is from 13.0 to 14.50 mol % in all structural units (100 mol %) in the fluorinated polymer (A).

TECHNICAL FIELD

The present invention relates to a production method for an ion exchangemembrane for alkali chloride electrolysis, and a production method foran alkali chloride electrolysis apparatus.

BACKGROUND ART

In alkali chloride electrolysis to produce an alkali hydroxide andchlorine by electrolyzing an aqueous solution of an alkali chloride(such as sodium chloride, potassium chloride or lithium chloride), anion exchange membrane having a layer made of a fluorinated polymerhaving carboxylic acid type functional groups on the cathode side andhaving a layer made of a fluorinated polymer having sulfonic acid typefunctional groups on the anode side, is used as a diaphragm.

Such an ion exchange membrane is produced, for example, by the followingmethod.

A method for producing an ion exchange membrane having a layercontaining a fluorinated polymer having carboxylic acid type functionalgroups and a layer containing a fluorinated polymer having sulfonic acidtype functional groups, by immersing an ion exchange membrane precursorfilm having a precursor layer containing a fluorinated polymer havinggroups convertible to carboxylic acid type functional groups, and aprecursor layer containing a fluorinated polymer having groupsconvertible to sulfonic acid type functional groups, in an alkalineaqueous solution comprising an alkali metal hydroxide, a water-solubleorganic solvent and water, to subject the groups convertible tocarboxylic acid type functional groups to hydrolysis treatment toconvert them to carboxylic acid type functional groups, and to subjectthe groups convertible to sulfonic acid type functional groups tohydrolysis treatment to convert them to sulfonic acid type functionalgroups (see e.g. Patent Documents 1 and 2).

PRIOR ART DOCUMENTS Patent Documents

Patent Document 1: Japanese Patent No. 4329339

Patent Document 2: WO2009/133902

DISCLOSURE OF INVENTION Technical Problem

However, the following two problems are likely to occur in alkalichloride electrolysis using an ion exchange membrane obtained by theconventional production method for an ion exchange membrane.

-   -   Variation in current efficiency at the time of electrolyzing an        alkali chloride tends to be large, and it is not possible to        carry out alkali chloride electrolysis stably.    -   At the time of electrolyzing an alkali chloride, the current        efficiency tends to decrease if the concentration of the alkali        hydroxide to be produced increases, i.e. alkali resistance will        be inadequate. If the current efficiency drops, substantial        demerits will result such as an increase in electric power        consumption rate, an increase in running costs due to an        increase in frequency of membrane replacement, a decrease in        chlorine quality due to an increase of oxygen concentration in        chlorine to be produced, etc.

It is an object of the present invention to provide a production methodfor an ion exchange membrane for an alkali chloride electrolysis,whereby an ion exchange membrane can be obtained which has high currentefficiency, little variation in current efficiency and high alkaliresistance, and a method for efficiently producing an alkali chlorideelectrolysis apparatus which has high current efficiency, littlevariation in current efficiency and high alkali resistance.

Solution to Problem

The present invention has the following embodiments.

[1] A production method for an ion exchange membrane for alkali chlorideelectrolysis, which is a method for producing an ion exchange membranefor alkali chloride electrolysis having a layer containing a fluorinatedpolymer having carboxylic acid type functional groups, by immersing anion exchange membrane precursor film having a precursor layer containinga fluorinated polymer having groups convertible to carboxylic acid typefunctional groups, in an alkaline aqueous solution containing an alkalimetal hydroxide, a water-soluble organic solvent and water, andsubjecting the groups convertible to carboxylic acid type functionalgroups to hydrolysis treatment to convert them to carboxylic acid typefunctional groups, wherein the concentration of the water-solubleorganic solvent is from 1 to 60 mass % in the alkaline aqueous solution(100 mass %), the temperature of the alkaline aqueous solution is atleast 40° C. and less than 80° C., and the proportion of structuralunits having the carboxylic acid type functional groups in thefluorinated polymer having the carboxylic acid type functional groups,is from 13.10 to 14.50 mol %, in all structural units (100 mol %) in thefluorinated polymer having the carboxylic acid type functional groups.[2] The production method for an ion exchange membrane for alkalichloride electrolysis according to [1], wherein the concentration of thewater-soluble organic solvent is from 5 to 50 mass % in the alkalineaqueous solution (100 mass %).[3] The production method for an ion exchange membrane for alkalichloride electrolysis according to [1] or [2], wherein the concentrationof the alkali metal hydroxide is from 1 to 60 mass % in the alkalineaqueous solution (100 mass %).[4] The production method for an ion exchange membrane for alkalichloride electrolysis according to any one of [1] to [3], wherein theconcentration of the alkali metal hydroxide is from 5 to 50 mass % inthe alkaline aqueous solution (100 mass %).[5] The production method for an ion exchange membrane for alkalichloride electrolysis according to any one of [1] to [4], wherein thealkali metal hydroxide is sodium hydroxide or potassium hydroxide.[6] The production method for an ion exchange membrane for alkalichloride electrolysis according to any one of [1] to [5], wherein thewater-soluble organic solvent is at least one member selected from thegroup consisting of aprotic water-soluble organic solvents, alcohols andaminoalcohols.[7] The production method for an ion exchange membrane for alkalichloride electrolysis according to [6], wherein the water-solubleorganic solvent is at least one member selected from the groupconsisting of dimethyl sulfoxide, methyl alcohol, ethyl alcohol, propylalcohol, 1-methoxy-2-propanol, triethanolamine, diethanolamine,isopropanolamine, triisopropanolamine, dimethylaminoethanol anddiethylaminoethanol.[8] The production method for an ion exchange membrane for alkalichloride electrolysis according to any one of [1] to [7], which is amethod for producing an ion exchange membrane for alkali chlorideelectrolysis further having a layer containing a fluorinated polymerhaving sulfonic acid type functional groups, wherein the ion exchangemembrane precursor film further has a precursor layer containing afluorinated polymer having groups convertible to sulfonic acid typefunctional groups, the ion exchange membrane precursor film is immersedin the alkaline aqueous solution, and the groups convertible tocarboxylic acid type functional groups are subjected to hydrolysistreatment and converted to carboxylic acid type functional groups, andat the same time, the groups convertible to sulfonic acid typefunctional groups are subjected to hydrolysis treatment and converted tosulfonic acid type functional groups.[9]. The production method for an ion exchange membrane for alkalichloride electrolysis according to any one of [1] to [8], wherein theion exchange membrane for alkali chloride electrolysis further has alayer comprising inorganic particles and a binder, on at least oneoutermost layer.[10] A production method for an alkali chloride electrolysis apparatus,which comprises mounting an ion exchange membrane for alkali chlorideelectrolysis obtained by the production method for an ion exchangemembrane for alkali chloride electrolysis as defined in any one of [1]to [9], in an electrolytic cell comprising a cathode and an anode, so asto partition the inside of the electrolytic cell into a cathode chamberon the cathode side and an anode chamber on the anode side.

Advantageous Effects of Invention

According to the production method for an ion exchange membrane foralkaline chloride electrolysis of the present invention, it is possibleto efficiently obtain an ion exchange membrane which has high currentefficiency, little variation in current efficiency and high alkaliresistance.

According to the production method for an alkali chloride electrolysisapparatus of the present invention, it is possible to efficientlyproduce an alkali chloride electrolysis apparatus which has high currentefficiency, little variation in current efficiency and high alkaliresistance.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic cross-sectional view showing an example of the ionexchange membrane for alkaline chloride electrolysis in the presentinvention.

FIG. 2 is a schematic diagram showing an example of the alkali chlorideelectrolysis apparatus in the present invention.

DESCRIPTION OF EMBODIMENTS

The following definitions of terms apply throughout this specificationincluding claims.

In this specification, a monomer represented by the formula (1) will bereferred to as a monomer (1). Monomers represented by other formulaewill be referred to in the same manner.

A “carboxylic acid type functional group” means a carboxylic acid group(—COOH) or a carboxylate (—COOM¹, wherein M¹ is an alkali metal or aquaternary ammonium).

A “sulfonic acid type functional group” means a sulfonic acid group(—SO₃H) or a sulfonate (—SO₃M², wherein M² is an alkali metal or aquaternary ammonium).

The “groups convertible to carboxylic acid type functional groups” meansgroups which can be converted to carboxylic acid type functional groupsby a known treatment such as hydrolysis treatment or acid formconversion treatment.

The “groups convertible to sulfonic acid type functional groups” meansgroups which can be converted to sulfonic acid type functional groups bya known treatment such as hydrolysis treatment or acid form conversiontreatment.

A “fluorinated polymer” means a polymer compound having fluorine atomsin the molecule.

A “perfluorocarbon polymer” means a polymer having all of hydrogen atomsbonded to carbon atoms in the polymer, substituted by fluorine atoms.Some of fluorine atoms in the perfluorocarbon polymer may be substitutedby chlorine atoms or bromine atoms.

A “monomer” means a compound having a polymerization reactivecarbon-carbon double bond.

The term “structural units” means units derived from a monomer andformed by polymerization of the monomer. Structural units may be unitsformed directly by a polymerization reaction of a monomer, or afterforming a polymer having certain structural units, such structural unitsmay be chemically changed to have part of the structural units convertedto another structure.

The term “primary particles” means the smallest particles to be observedby a scanning electron microscope (SEM). Further, the term “secondaryparticles” means particles having primary particles agglomerated.

An “ion exchange membrane precursor film” is a film before beingsubjected to hydrolysis treatment, and means a film having a precursorlayer containing a fluorinated polymer having groups convertible tocarboxylic acid type functional groups. In the ion exchange membraneprecursor film, carboxylic acid type functional groups in the ionexchange membrane are in the state of groups which can be converted tocarboxylic acid type functional groups. Further, in a case where the ionexchange membrane has a precursor layer containing a fluorinated polymerhaving sulfonic acid type functional groups, also such sulfonic acidtype functional groups are in the state of groups which can be convertedto sulfonic acid type functional groups.

The “precursor layer” means a layer made of a fluorinated polymer havinggroups convertible to carboxylic acid type functional groups. Theprecursor layer may contain a layer made of a fluorinated polymer havinggroups convertible to sulfonic acid type functional groups. In theprecursor layer, the carboxylic acid type functional groups in the ionexchange membrane are in the state of groups which can be converted tocarboxylic acid type functional groups, and sulfonic acid typefunctional groups are in the state of groups which can be converted tosulfonic acid type functional groups.

<Ion Exchange Membrane for Alkali Chloride Electrolysis>

The ion exchange membrane for alkali chloride electrolysis (hereinaftersimply referred to also as the “ion exchange membrane”) obtainable bythe production method of the present invention, has a layer (hereinaftersimply referred to also as a “layer (C)”) containing a fluorinatedpolymer having carboxylic acid type functional groups (hereinaftersimply referred to also as a “fluorinated polymer (A)”).

The ion exchange membrane may further have a layer (hereinafter simplyreferred to also as a “layer (S)”) containing a fluorinated polymerhaving sulfonic acid type functional groups (hereinafter simply referredto also as a “fluorinated polymer (B)”).

The ion exchange membrane may further have a layer comprising inorganicparticles and a binder (hereinafter simply referred to also as an“inorganic particle layer”) on at least one outermost layer.

In the ion exchange membrane, a reinforcing material may be embeddedbetween the layer (S) and the layer (C), in the layer (S), or in thelayer (C).

FIG. 1 is a schematic cross-sectional view showing an example of the ionexchange membrane in the present invention.

The ion exchange membrane 1 has a first inorganic particle layer 10, alayer (C) 12 containing a fluorinated polymer (A), a layer (S) 14containing a fluorinated polymer (B), and a second inorganic particlelayer 16 in this order.

The layer (S) 14 may have a reinforcing material 18, and the reinforcingmaterial 18 is embedded between a first layer (S1) 14 a and a secondlayer (S2) 14 b.

The ion exchange membrane 1 will be disposed so that, in an electrolyticcell, the first inorganic particle layer 10 faces the cathode, and thesecond inorganic particle layer 16 faces the anode.

The shape and size of the ion exchange membrane 1 may be suitablydetermined depending on an electrolytic cell in which the ion exchangemembrane 1 is to be mounted.

(Layer (C))

The layer (C) containing a fluorinated polymer (A) expresses highcurrent efficiency. The layer (C) may be a layer having a reinforcingmaterial embedded therein. As the layer (C), from the viewpoint of theelectrolytic performance, a layer composed solely of the fluorinatedpolymer (A) is preferred which does not contain a material other thanthe fluorinated polymer (A) such as a reinforcing material. The layer(C) may be single layered or multilayered. A multi-layered layer (C)may, for example, be one with such a construction that in the respectivelayers, the types of structural units constituting the fluorinatedpolymers (A) or the proportions of structural units having carboxylicacid type functional groups are different.

The fluorinated polymer (A) is one obtained by subjecting groupsconvertible to carboxylic acid type functional groups in a fluorinatedpolymer (hereinafter simply referred to also as a “fluorinated polymer(A′)”) having groups convertible to carboxylic acid type functionalgroups, to hydrolysis treatment to convert them to carboxylic acid typefunctional groups.

As the fluorinated polymer (A), preferred is a fluorinated polymer(hereinafter simply referred to also as a “fluorinated polymer (A1)”obtained by subjecting a fluorinated polymer (hereinafter simplyreferred to also as a “fluorinated polymer (A′1)”) having structuralunits based on the following monomer (1) and structural units based onthe following monomer (2) or monomers (2′), to hydrolysis treatment toconvert Y to —COOM (wherein M is an alkali metal).CF₂═CX¹X²  (1)CF₂═CF(OCF₂CFX³)_(n)O(CF₂)_(m)Y  (2)CF₂═CF(CF₂)_(p)—(O)_(q)—(CF₂CFX⁴)_(r)—(O)_(s)—(CF₂)_(t)—(CF₂CFX⁵)_(u)—Y  (2′)

X¹ and X² are each independently a fluorine atom, a chlorine atom or atrifluoromethyl group, and from the viewpoint of the chemical durabilityof the ion exchange membrane, a fluorine atom is preferred.

The monomer (1) may, for example, be CF₂═CF₂, CF₂═CFCl, CF₂═CFCF₃, etc.,and from the viewpoint of chemical durability of the ion exchangemembrane, CF₂═CF₂ is preferred.

X³ is a fluorine atom or a trifluoromethyl group.

m is an integer of from 1 to 5.

n is 0 or 1.

X⁴ is a fluorine atom or a trifluoromethyl group. X⁵ is a fluorine atomor a trifluoromethyl group. In a case where both of X⁴ and X⁵ arepresent in one molecule, they may be the same or different.

p is 0 or 1, q is 0 or 1, r is an integer of from 0 to 3, s is 0 or 1, tis an integer from 0 to 12, u is an integer of from 0 to 3, and 1≤r+u.

Y is a group convertible to a carboxylic acid type functional group byhydrolysis. Y is preferably —COOR¹ (wherein R¹ is a C₁₋₄ alkyl group),—CN or —COX⁶ (wherein X⁶ is a halogen atom), more preferably —COOR¹,particularly preferably —COOCH₃.

As the monomer (2), for example, the following compounds may bementioned, and from the viewpoint of ion selectivity of the ion exchangemembrane, and industrial productivity of the monomer, a monomer whereinp=0, q=1, r=1, s=0 to 1, t=1 to 3 and u=0 to 1, is preferred, and amonomer represented by CF₂═CF—O—CF₂CF₂—CF₂—COOCH₃ is particularlypreferred.CF₂═CFOCF₂CF(CF₃)OCF₂CF₂COOCH₃,CF₂═CFOCF₂CF₂COOCH₃,CF₂═CFOCF₂CF₂CF₂COOCH₃,CF₂═CFOCF₂CF₂OCF₂CF₂COOCH₃,CF₂═CFOCF₂CF₂CF₂CF₂CF₂COOCH₃,CF₂═CFOCF₂CF(CF₃)OCF₂CF₂CF₂COOCH₃.

The lower limit for the proportion of structural units having carboxylicacid type functional groups in the fluorinated polymer (A), is 13.10 mol%, preferably 13.15 mol %, more preferably 13.20 mol %, furtherpreferably 13.25 mol %, among all structural units (100 mol %) in thefluorinated polymer (A). The upper limit for the proportion ofstructural units having carboxylic acid type functional groups in thefluorinated polymer (A), is 14.50 mol %, preferably 14.45 mol %, morepreferably 14.40 mol %, further preferably 14.35 mol %, among allstructural units (100 mol %) in the fluorinated polymer (A).

When the proportion of structural units having carboxylic acid typefunctional groups in the fluorinated polymer (A) is at least the lowerlimit value in the above range, it is possible to obtain an ion exchangemembrane having high alkali resistance at the time of electrolyzing analkali chloride. When the proportion of structural units havingcarboxylic acid type functional groups in the fluorinated polymer (A) isat most the upper limit value in the above range, it is possible toobtain an ion exchange membrane having high current efficiency at thetime of electrolyzing an alkali chloride.

In order to bring the proportion of structural units having carboxylicacid type functional groups in the fluorinated polymer (A) to be withinthe above range, a fluorinated polymer (A′) is employed which has groupsconvertible to carboxylic acid type functional groups in the sameproportion as the proportion of structural units having carboxylic acidtype functional groups in the fluorinated polymer (A).

A fluorinated polymer (A1) is obtainable by hydrolyzing a fluorinatedpolymer (A′1) obtained by copolymerizing the monomer (1) and the monomer(2). The polymerization method at the time of copolymerizing the monomer(1) and the monomer (2) is not particularly limited, and may be any ofsolution polymerization, emulsion polymerization, suspensionpolymerization, and bulk polymerization, but solution polymerization oremulsion polymerization is preferred, and solution polymerization ismore preferred.

In the solution polymerization, the respective monomers may be chargedall at once, or may be sequentially or continuously added and reacted.From the viewpoint of uniformizing the composition of the fluorinatedpolymer to be produced, it is preferred that the respective monomers aresequentially or continuously added to react them while controlling theconcentrations of the respective monomers to be constant.

The fluorinated polymer (A) thus obtained by the solution polymerizationtends to be uniform in the composition of the formed fluorinatedpolymer, as compared with other polymerization methods. With thefluorinated polymer (A) obtained by the solution polymerization, sincethe composition is uniform, and polymer molecules out of the optimumcomposition tend to be less, it becomes possible to express currentefficiency stably under a wide operating region.

The thickness of the layer (C) is preferably from 5 to 50 μm, morepreferably from 10 to 35 μm. When the thickness of the layer (C) is atleast the lower limit value in the above range, it is possible tosuppress concentration of an alkali chloride at the cathode side, whichtransmits from the anode side, and it is possible to maintain thequality of a an aqueous alkali hydroxide solution to be good as aproduct. When the thickness of the layer (C) is at most the upper limitvalue in the above range, the electric resistance of the ion exchangemembrane tends to be low, whereby it is possible to lower theelectrolytic voltage.

(Layer (S))

The layer (S) containing a fluorinated polymer (B) contributes tomaintaining the mechanical strength of the ion exchange membrane. Fromthe viewpoint of increasing the mechanical strength of the ion exchangemembrane, it is preferred that a reinforcing material is embedded in thelayer (S). In the case of embedding a reinforcing material, by embeddingit in the layer (S) instead of in the layer (C), it is possible toobtain the reinforcing effect without affecting the electrolysisperformance. The layer (S) may be single layered or multilayered. Amulti-layered layer (S) may, for example, be one having such aconstruction that in the respective layers, the types of structuralunits constituting the fluorinated polymer (B) or the proportions ofstructural units having sulfonic acid type functional groups aredifferent.

In the case of embedding a reinforcing material in the layer (S), it ispreferred that the layer (S) is made to be multilayered, and areinforcing material is inserted to one interlayer between such layersduring the production, followed by lamination bonding to let thereinforcing material be embedded.

The fluorinated polymer (B) is one obtained by subjecting groupsconvertible to sulfonic acid type functional groups in a fluorinatedpolymer (hereinafter simply referred to also as a “fluorinated polymer(B′)”) having the groups convertible to sulfonic acid type functionalgroups, to hydrolysis treatment to convert them to sulfonic acid typefunctional groups.

As the fluorinated polymer (B), preferred is a fluorinated polymer(hereinafter simply referred to also as a “fluoropolymer (B1)”) obtainedby subjecting a fluorinated polymer (hereinafter simply referred to alsoas a “polymer (B′1)”) having structural units based on the followingmonomer (1) and structural units based on the following monomer (3), tohydrolysis treatment, to convert Z to —SO₃M (wherein M is an alkalimetal).CF₂═CX¹X²  (1)CF₂═CF(OCF₂CFX⁴)_(s)O(CF₂)_(t)Z  (3)

X¹ and X² are each independently a fluorine atom, a chlorine atom or atrifluoromethyl group, and from the viewpoint of chemical durability ofthe ion exchange membrane, a fluorine atom is preferred.

The monomer (1) may, for example, be CF₂═CF₂, CF₂═CFCl, CF₂═CFCF₃, etc.,and from the viewpoint of chemical durability of the ion exchangemembrane, CF₂═CF₂ is preferred.

X⁴ is a fluorine atom or a trifluoromethyl group.

s is an integer of from 1 to 3.

t is an integer of from 0 to 3.

Z is a group which can be converted to a sulfonic acid functional groupby hydrolysis. Z is preferably —SO₂X⁵ (wherein X⁵ is a fluorine atom, achlorine atom or a bromine atom) or —SO₂R² (wherein R² is a C₁₋₄ alkylgroup), more preferably —SO₂X⁵, particularly preferably —SO₂F.

As the monomer (3), from the viewpoint of the strength intensity of theion exchange membrane, and industrial productivity of the monomer, thefollowing compounds are preferred.CF₂═CFOCF₂CF(CF₃)OCF₂CF₂CF₂SO₂F,CF₂═CFOCF₂CF(CF₃)OCF₂CF₂SO₂F,CF₂═CFOCF₂CF₂CF₂SO₂F,CF₂═CFOCF₂CF₂SO₂F.

The proportion of structural units having sulfonic acid type functionalgroups in the fluorinated polymer (B) is preferably from 13.10 to 30.50mol %, more preferably from 15.00 to 18.00 mol %, among all structuralunits (100 mol %) in the fluorinated polymer (B).

When the proportion of structural units having sulfonic acid typefunctional groups in the fluorinated polymer (B) is at least the lowerlimit value in the above range, the electric resistance of the ionexchange membrane tends to be lower, whereby it is to possible to lowerthe electrolytic voltage. When the proportion of structural units havingsulfonic acid type functional groups in the fluorinated polymer (B) isat most the upper limit value in the above range, the synthesis of afluoropolymer (B) with a high molecular weight will be easy, and it ispossible to prevent swelling of the fluorinated polymer (B).

With a view to preventing peeling between the layer (C) and the layer(S), the difference between the proportion X_(c) of structural unitshaving carboxylic acid type functional groups in the fluorinated polymer(A) constituting the layer (C) adjacent to the layer (S) and theproportion X_(s) of structural units having sulfonic acid typefunctional groups in the fluorinated polymer (B) constituting the layer(S) adjacent to the layer (C), is preferably small.

In the case of embedding a reinforcing material in the layer (S), it ispreferred to make the layer (S) to be multilayer as shown in FIG. 1 andembed a reinforcing material in between the layers. In this case, fromsuch a viewpoint that it is easy to reduce the electrolysis voltage, theproportion X_(s2) of structural units having sulfonic acid typefunctional groups in the fluorinated polymer (B) constituting the secondlayer (S2) (14 b in FIG. 1) on the anode side of the reinforcingmaterial, is preferably equal to or higher than the proportion X_(s1) ofstructural units having sulfonic acid type functional groups in thefluorinated polymer (B) constituting the first layer (S1) (14 a inFIG. 1) on the cathode side of the reinforcing material.

In a case where the layer (S) is made to be multilayer, the fluorinatedpolymers (B) forming the respective layers may be the same or different.

The thickness of the layer (S) is preferably from 55 to 200 μm, morepreferably from 70 to 160 μm. When the thickness of the layer (S) is atleast the lower limit value in the above range, the mechanical strengthof the ion exchange membrane will be sufficient so as to be durableagainst long-term electrolysis. When the total thickness of the layer(S) is at most the upper limit value in the above range, it is possibleto sufficiently lower the electrolysis voltage.

In the case of embedding a reinforcing material in the layer (S), thethickness of the second layer (S2) on the anode side of the reinforcingmaterial is preferably from 10 to 60 μm. When the thickness of thesecond layer (S2) is at least the lower limit value in the above range,the reinforcing material can easily be embedded in the layer (S) andinterlayer peeling can be suppressed. When the thickness of the secondlayer (S2) is at most the upper limit value in the above range, theelectric resistance of the ion exchange membrane tends to be low,whereby the electrolytic voltage can be made low.

In the case of embedding a reinforcing material in the layer (S), thethickness of the first layer (S1) on the cathode side of the reinforcingmaterial is preferably from 45 to 140 μm, more preferably from 60 to 100μm.

(Inorganic Particle Layer)

The ion exchange membrane may have, on at least one outermost surfacelayer, an inorganic particle layer comprising inorganic particles(hereinafter referred to also as “inorganic particles (P)”) and thebinder.

By providing the inorganic particle layer on the outermost surface layerof the ion exchange membrane, it is possible to suppress adhesion of gasto the surface of the ion exchange membrane, whereby at the time ofelectrolyzing an alkali chloride solution, it is possible to suppressincrease of the electrolytic voltage. Further, since the inorganicparticle layer contains a binder, it is excellent in falling resistanceof inorganic particles (P), whereby inorganic particles (P) are lesslikely to fall off, even if friction has occurred with other members,etc., and the effect to suppress adhesion of gas can be stably obtained.

The inorganic particles (P) are preferably ones which are excellent incorrosion resistance to the alkali chloride aqueous solution, etc., andwhich have hydrophilic properties. Specifically, preferred is at leastone member selected from the group consisting of oxides, nitrides andcarbides of Group 4 elements and Group 14 elements in the periodictable, more preferred is SiO2, SiC, ZrO₂ or ZrC, and particularlypreferred is ZrO₂.

The average primary particle diameter of the inorganic particles (P) ispreferably from 0.01 to 1 μm, more preferably from 0.02 to 0.4 μm. Whenthe average primary particle diameter of the inorganic particles (P) isat least the lower limit value in the above range, unevenness due toaggregation will be less. When the average primary particle diameter ofthe inorganic particles (P) is at most the upper limit value in theabove range, unevenness due to poor dispersion will be less.

The average secondary particle diameter of the inorganic particles (P)is preferably from 0.5 to 1.5 μm, more preferably from 0.7 to 1.3 μm.When the average secondary particle diameter of the inorganic particles(P) is at least the lower limit value in the above range, a high gasadhesion suppressing effect will be obtained. When the average secondaryparticle diameter of the inorganic particles (P) is at most the upperlimit value in the above range, falling resistance of inorganicparticles (P) will be excellent.

As the binder contained in the inorganic particle layer (hereinafterreferred to also as a “first inorganic particle layer”) provided on thelayer (C) side surface of the ion exchange membrane, preferred is onewhich is excellent in corrosion resistance to the alkali hydroxideaqueous solution, etc. and which has hydrophilic properties, morepreferred is a fluorinated polymer (H) having carboxylic acid groups orsulfonic acid groups, and further preferred is a fluorinated polymer (H)having sulfonic acid groups. The fluorinated polymer (H) may be ahomopolymer of a monomer having a carboxylic acid group or sulfonic acidgroup, or a copolymer of a monomer having a carboxylic acid group orsulfonic acid group and a monomer copolymerizable therewith.

As the fluorinated polymer (H) having carboxylic acid groups, a polymerobtained by subjecting a copolymer having structural units based on themonomer (1) and structural units based on the monomer (2) to hydrolysistreatment, followed by acid form conversion treatment to convert Y to—COOH, may be mentioned.

As the fluorinated polymer (H) having sulfonic acid groups, a polymerobtained by subjecting a copolymer having structural units based on themonomer (1) and structural units based on the monomer (3) to hydrolysistreatment, followed by acid form conversion treatment to convert Z to—SO₃H, may be mentioned.

The mass ratio of the binder (hereinafter referred to also as the“binder ratio”) to the total mass of the inorganic particles (P) and thebinder in the first inorganic particle layer Is preferably from 0.15 to0.30, more preferably from 0.15 to 0.25, further preferably from 0.16 to0.20. When the binder ratio in the first inorganic particle layer is atleast the lower limit value in the above range, falling resistance ofthe inorganic particles (P) will be excellent. When the binder ratio inthe first inorganic particle layer is at most the upper limit value inthe above range, a high gas adhesion suppressing effect will beobtainable.

As the inorganic particle layer to be provided on the layer (S) sidesurface of the ion exchange membrane (hereinafter referred to also asthe “second inorganic particle layer”), a known hydrophilic layer (gasrelease layer) to be provided on the anode side of an ion exchangemembrane for use in electrolysis of an alkali chloride aqueous solution,may be employed.

As the binder contained in the inorganic particle layer to be providedon the layer (S) side surface of the ion exchange membrane, a knownbinder for use in the known hydrophilic layer (gas release layer) to beprovided on the anode side, may be employed, and, for example,methylcellulose, etc. may be mentioned. The binder used in the inorganicparticle layer provided on the layer (C) side surface of the ionexchange membrane may also be used.

(Reinforcing Material)

The reinforcing material is a material used to improve the strength ofthe ion exchange membrane, and is a material formed from a sacrificialyarn optionally contained and a reinforcing yarn derived from areinforcing fabric formed by dissolution of at least a part of asacrificial yarn in the reinforcing fabric by immersing an ion exchangemembrane precursor film having a precursor layer containing afluorinated polymer having the reinforcing fabric embedded therein in analkaline aqueous solution in the process for the production of an ionexchange membrane. In a case where a part of a sacrificial yarn has beendissolved, the reinforcing material is a material composed of thereinforcing yarn and the sacrificial yarn remaining as undissolved, andin a case where all of the sacrificial yarn has been dissolved, it iscomposed solely of the reinforcing yarn. That is, the reinforcingmaterial is a material formed from the reinforcing yarn and optionallycontained sacrificial yarn. The reinforcing material is embedded in theion exchange membrane, and may be formed by immersing a precursor filmmade of a fluorinated polymer having a reinforcing fabric embeddedtherein, in an aqueous alkaline solution.

The reinforcing fabric as a raw material for the reinforcing material isa fabric to be used as a raw material for the reinforcing material forimproving the strength of an ion exchange membrane, and may, forexample, be a woven fabric, a nonwoven fabric, fibril, a porous body,etc., and from the viewpoint of mechanical strength, a woven fabric ispreferred. In a case where the reinforcing fabric is a woven fabric,such a woven fabric is preferably one having a reinforcing yarn and asacrificial yarn woven. The reinforcing yarn and the sacrificial yarn inthe reinforcing fabric are woven as warp and weft, respectively, andthese warp and weft are orthogonal in the case of a usual weaving methodsuch as plain weave. As the material for the reinforcing yarn, afluorinated polymer such as polytetrafluoroethylene (hereinafterreferred to also as PTFE) may be mentioned. As the material for thesacrificial yarn, polyethylene terephthalate (hereinafter referred to asPET) may be mentioned.

(Current Efficiency of Ion Exchange Membrane)

The current efficiency of the ion exchange membrane is preferably atleast 96.0%. When the current efficiency of the ion exchange membrane ishigh, it is possible to efficiently electrolyze an alkali chloride.

The current efficiency of the ion exchange membrane is measured by themethod described in Examples. However, the apparatus used for themeasurement may be replaced by a device having the same function.

<Production Method for Ion Exchange Membrane for Alkali ChlorideElectrolysis>

The production method for an ion exchange membrane of the presentinvention essentially requires the following step (b), and may have thefollowing step (a), as the case requires.

Step (a): A step of obtaining an ion exchange membrane precursor filmhaving a precursor layer (C′) containing a fluorinated polymer (A′).

Step (b): A step of immersing the ion exchange membrane precursor filmobtained in step (a) in an alkaline aqueous solution containing analkali metal hydroxide, a water-soluble organic solvent and water, andsubjecting groups convertible to carboxylic acid type functional groupsto hydrolysis treatment to convert them to the carboxylic acid typefunctional groups.

(Step (a))

In the step (a), an ion exchange membrane precursor film having aprecursor layer (C′) containing a fluorinated polymer (A′) is produced.The ion exchange membrane precursor film may further has a precursorlayer (S′) containing a fluorinated polymer (B′), as the case requires.

The ion exchange membrane precursor film may be produced by a knownmethod using a fluorinated polymer (A′). In a case where it further hasa precursor layer (S′), it may be produced by a known method usingfurther a fluorinated polymer (B′). In a case where it further has aprecursor layer (S′), or in a case where the precursor layer (C′) orprecursor layer (S′) is multilayered, it can be produced by laminatingthe respective layers.

As the fluorinated polymer (A′), the above-mentioned fluorinated polymer(A′1) is preferred. As the fluorinated polymer (B′), the above-mentionedfluorinated polymer (B′1) is preferred.

As the fluorinated polymer (A′), one whereby the proportion ofstructural units having carboxylic acid type functional groups in afluorinated polymer (A) obtainable after the hydrolysis treatment willbe from 13.10 to 14.50 mol % among all structural units (100 mol %) inthe fluorinated polymer (A), is employed. As the fluorinated polymer(A′), preferred is one whereby the proportion of structural units havingcarboxylic acid type functional groups in the fluorinated polymer (A)will be within the above range.

When the proportion of structural units having carboxylic acid typefunctional groups in the fluorinated polymer (A) obtainable afterhydrolysis treatment, is at least the lower limit value in the aboverange, it is possible to obtain an ion exchange membrane having highalkali resistance at the time of electrolyzing an alkali chloride. Whenthe proportion of structural units having carboxylic acid typefunctional groups in the fluorinated polymer (A) obtainable afterhydrolysis treatment is at most the upper limit value in the aboverange, it is possible to obtain an ion exchange membrane having highcurrent efficiency at the time of electrolyzing an alkali chloride.

(Step (b))

In step (b), the ion exchange membrane precursor film is immersed in analkaline aqueous solution containing an alkali metal hydroxide, awater-soluble organic solvent and water, to subject groups convertibleto carboxylic acid type functional groups to hydrolysis treatment toconvert them to the carboxylic acid type functional groups. In a casewhere the ion exchange membrane precursor film further has a precursorlayer (S′), at the same time as converting groups convertible tocarboxylic acid type functional groups to the carboxylic acid typefunctional groups, groups convertible to sulfonic acid type functionalgroups are subjected to hydrolysis treatment to convert them to thesulfonic acid type functional groups.

The concentration of the water-soluble organic solvent is from 1 to 60mass %, preferably from 3 to 55 mass %, more preferably from 5 to 50mass %, in the alkaline aqueous solution (100 mass %). If theconcentration of the water-soluble organic solvent is less than thelower limit value, the hydrolysis treatment speed will be low, wherebythe productivity will be low. If the concentration of the water-solubleorganic solvent exceeds the upper limit value, the polymer tends to beexcessively swelled, the resistivity of the polymer will be low, thewater content will increase, and the current efficiency will decreasedue to the aforementioned mechanism.

The concentration of the alkali metal hydroxide is preferably from 1 to60 mass %, more preferably from 3 to 55 mass %, further preferably from5 to 50 mass %, in the alkaline aqueous solution (100 mass %). When theconcentration of the alkali metal hydroxide is within the above range,the hydrolysis treatment will swiftly proceed.

The concentration of water is preferably from 39 to 80 mass %, morepreferably from 45 to 70 mass %, in the alkaline aqueous solution (100mass %).

The temperature of the alkaline aqueous solution in which the ionexchange membrane precursor film is to be immersed, is at least 40° C.and less than 80° C., preferably from 42 to 78° C., more preferably from44 to 76° C. When the temperature of the alkaline aqueous solution is atleast the lower limit value in the above range, the electrolysis voltagewill be suppressed to be low, and the hydrolysis treatment will beswiftly completed, whereby the productivity of the ion exchange membranewill be improved. When the temperature of the alkaline aqueous solutionis at most the upper limit value in the above range, it is possible toobtain an ion exchange membrane having high current efficiency andlittle variation in current efficiency at the time of electrolyzing analkali chloride.

If the temperature of the alkaline aqueous solution is less than thelower limit value, the speed of hydrolysis tends to be low, whereby theproductivity will be low. If the temperature of the alkaline aqueoussolution exceeds the upper limit value, the polymer tends to beexcessively swelled, the resistivity of the polymer tends to be low, thewater content tends to increase, and the current efficiency willdecrease due to the mechanism as described above.

The temperature for hydrolysis also affects variation in currentefficiency. When the temperature for hydrolysis is low, the hydrolysisreaction tends to be slow, the polymer is less likely to be swelled, andion paths (ion channels) in the polymer structure are likely to beuniformly formed over time. However, if the temperature for hydrolysisis high, the hydrolysis reaction tends to proceed rapidly, the polymertends to be excessively swelled, and ion channels in the polymerstructure tend to be non-uniformly formed. For such reasons, when thetemperature of the alkaline aqueous alkali solution is at most the upperlimit value, it is possible to obtain an ion exchange membrane havinglittle variation in the current efficiency.

The time for immersing the ion exchange membrane precursor film in analkaline aqueous solution is preferably from 5 minutes to 3 hours, morepreferably within 2 hours, further preferably within 1 hour.

As the alkali metal hydroxide, sodium hydroxide or potassium hydroxideis preferred, and potassium hydroxide is more preferred. As the alkalimetal hydroxide, one type may be used alone, or two or more types may beused in combination.

The water-soluble organic solvent may, for example, be an aproticwater-soluble organic solvent (dimethyl sulfoxide, etc.), an alkylalcohol (methyl alcohol, ethyl alcohol, propyl alcohol, etc.), an alkoxyalcohol (1-methoxy-2-propanol, 1-ethoxy 2-propanol,1-methoxy-2-methyl-2-propanol, 1-propoxy-2-propanol,1-isopropoxy-2-propanol, 2-ethoxy-1-propanol, 2,3-ethoxy-1-propanol,2-methoxy-1-propanol, 1-butoxy-2-propanol, etc.), an aryloxy alcohol(2-phenoxy-1-propanol), or an amino alcohol (triethanolamine,diethanolamine, isopropanolamine, triisopropanolamine,dimethylaminoethanol, diethylaminoethanol, etc.). Here, in theexemplification of the alkoxy alcohol or the aryloxy alcohol, a solventhaving its propanol portion replaced by another alcohol (such asethanol, butanol, etc.) may also be mentioned as a preferred solvent.

As the water-soluble organic solvent, with a view to swelling the ionexchange membrane and promoting the hydrolysis reaction rate, at leastone member selected from the group consisting of the aproticwater-soluble organic solvent, the alcohol and the amino alcohol, ispreferred, at least one member selected from the group consisting ofdimethyl sulfoxide, methyl alcohol, ethyl alcohol, propyl alcohol,1-methoxy-2-propanol, triethanolamine, diethanolamine, isopropanolamine,triisopropanolamine, dimethylaminoethanol and diethylaminoethanol, ismore preferred, and 1-methoxy-2-propanol or dimethyl sulfoxide isfurther preferred. As the water-soluble organic solvent, one type may beused alone, or two or more types may be used in combination.

The alkaline aqueous solution is required only to contain water, analkali metal hydroxide and a water-soluble organic solvent, and it maybe uniform or non-uniformly phase separated at the temperature for thehydrolysis, but is preferably uniformly compatibilized.

In order to efficiently produce an ion exchange membrane for alkalichloride electrolysis, which has high current efficiency, less variationin current efficiency and high alkali-resistance, the composition of thealkaline aqueous solution to be used for hydrolysis, the temperature forhydrolysis, and the proportion of structural units having carboxylicacid type functional groups, may be adjusted. For example, as comparedwith one having a low functional group content, a polymer having a highfunctional group content tends to have a high water content, whereby theresistivity tends to be low, and therefore, adjustment may be made byreducing the amount of the organic solvent, or lowering the temperaturefor hydrolysis.

Thus, by controlling the adjustment factors to correspond to theabove-described ranges, it is possible to efficiently produce an ionexchange membrane for alkaline chloride electrolysis which has highcurrent efficiency and alkali resistance at the time of electrolyzing analkali chloride.

(Method for Forming Inorganic Particle Layer)

In a case where the ion exchange membrane precursor film furthercontains an inorganic particle layer, such an inorganic particle layermay be formed by applying a coating solution (hereinafter referred toalso as a “coating solution (D)”) comprising inorganic particles (P), abinder and a dispersion medium, to the surface of the layer (C) or layer(S) of an ion exchange membrane, or the layer (C′) or layer (S′) of anion exchange membrane precursor film, followed by removing thedispersion medium by e.g. heating, for drying.

Otherwise, the inorganic particle layer may be formed by applying apast-form coating liquid (D) comprising inorganic particles (P), abinder and a dispersion medium, to a transfer substrate to form aninorganic particle layer, which is then transferred to the surface ofthe layer (C) or layer (S) of an ion exchange membrane, or the layer(C′) or layer (S′) of an ion-exchange membrane precursor film.

As the preparation method for the coating liquid (D), preferred is amethod of mixing inorganic particles (P), a binder and a dispersionmedium, and stirring them by means of a ball mill to be uniform,followed by dispersion treatment by means of a bead mill. By using sucha method, it is easy to control the average secondary particle diameterof inorganic particles (P) to be within the aforementioned range.

The average secondary particle diameter of inorganic particles (P) inthe coating liquid (D) can be controlled by adjusting the averageprimary particle diameter of inorganic particles (P), the treating timefor dispersion treatment, etc.

As the dispersion medium, in a case where the binder is a fluorinatedpolymer (H) having sulfonic acid groups, an alcohol solvent (ethanol,isopropyl alcohol, etc.) is preferred.

Further, as the dispersion medium, an aprotic polar solvent such asdimethyl sulfoxide, formamide, N,N-dimethylacetamide, orN,N-dimethylformamide, may be used. As the aprotic polar solvent,preferred is one having a boiling point of at least 140° C. and at mostthe melting point of the fluorinated polymer (A) and the fluorinatedpolymer (B), and a melting point of at most 25° C.

In the case of using an aprotic polar solvent, a coating liquid (D)having the aprotic polar solvent incorporated, may be prepared andapplied, or a coating liquid (D) using a dispersion medium (such as analcohol type solvent) other than an aprotic polar solvent may beprepared and applied, and then, the aprotic polar solvent may beapplied.

The content of the dispersion medium in the coating liquid (D) (100 mass%) is preferably from 30 to 95 mass %, more preferably from 70 to 90mass %. When the content of the dispersion medium is within the aboverange, the dispersibility of the binder will be good, and the viscositywill also be proper, such being suitable for the case of applying thecoating liquid (D) by a spray method.

In a case where an aprotic polar solvent is to be used, the content ofthe aprotic polar solvent in the coating liquid (D) (100 mass %) ispreferably from 1 to 70 mass %, more preferably from 10 to 50 mass %.

As a method for applying the coating liquid (D), a known coating methodmay be employed, and, for example, a spray method, a roll coater method,etc. may be mentioned, but a spray method is preferred. In a case whereprior to step (b), a later-described step (c) is to be carried out, fromsuch a viewpoint that adhesion of the coating solution (D) becomesbetter, a spray method is preferred, and it is particularly preferred toreduce the amount of air in the spray method.

The heating method for removing the dispersion medium may, for example,be a method of using a heating roll, a method of using an oven, etc.,and industrially, a method for conducting heat treatment continuously bya roll press machine having a heated roll is preferred.

In the case of using a roll press, the pressure to be applied ispreferably a linear pressure of at most 0.2 MPa from the viewpoint ofreduction of the power.

The heating temperature for removing the dispersion medium is preferablyat least 30° C., more preferably at least the boiling point of thedispersion medium to be used. If the heating temperature is lower thanthe boiling point of the dispersion medium, the dispersing medium islikely to remain on the surface of the ion exchange membrane, butdepending on the type of the dispersion medium, even by heating at atemperature of lower than the boiling point, the dispersion medium canbe sufficiently volatilized, from the relationship of the vaporpressure.

Further, the heating temperature is preferably less than the meltingpoint of the fluorinated polymer (A) and the fluorinated polymer (B).Thus, it becomes easy to prevent the film thickness from becominguneven.

(One Embodiment of Production Method for Ion Exchange Membrane)

Now, an example for an embodiment of the production method for an ionexchange membrane of the present invention will be described withreference to an example of the ion exchange membrane 1 in FIG. 1.

The ion exchange membranes 1 can be produced, for example, by a methodhaving the following steps (a) to (c).

Step (a): A step of obtaining a second inorganic particle layer-attachedion exchange membrane precursor film, which has a second inorganicparticle layer 16, a second precursor layer (S′2) containing afluorinated polymer (B′), a reinforcing fabric, a first precursor layer(S′1) containing a fluorinated polymer (B′) and a precursor layer (C′)containing a fluorinated polymer (A′) in this order.

Step (b): A step of immersing the second inorganic particlelayer-attached ion exchange membrane precursor film, in an alkalineaqueous solution comprising water, potassium hydroxide and dimethylsulfoxide, subjecting groups convertible to carboxylic acid typefunctional groups and groups convertible to sulfonic acid typefunctional groups to hydrolysis treatment to convert them to carboxylicacid type functional groups and sulfonic acid functional groups, and atthe same time, dissolving at least part of sacrificial yarns in thereinforcing fabric, to obtain a composite film which has the secondinorganic particle layer 16, a second layer (S2) 14 b, the reinforcingmaterial 18, a first layer (S1) 14 a and a layer (C) 12 in this order.

Step (c): A step of applying a coating liquid comprising inorganicparticles (P), a binder and a dispersing medium on the surface of thelayer (C) 12 of the composite film to form a first inorganic particlelayer 10, to obtain an ion exchange membrane 1.

Step (a):

In the following, the above steps (a) to (c) will be described in moredetail.

As the method for obtaining an ion exchange membrane precursor film, forexample, a method having the following means (i) to (iv) may bementioned. However, as the fluorinated polymer (A′), one whereby theproportion of structural units having carboxylic acid type functionalgroups in a fluorinated polymer (A) obtainable after hydrolysistreatment will be in the above-mentioned range, is to be employed.

(i) A means to obtain a laminate film having a layer (C′) of afluorinated polymer (A′) and a layer of a fluorinated polymer (B′)laminated by means of a film die for co-extrusion.

(ii) A means to obtain a single-layer film of a fluorinated polymer (B′)by means of a film die for monolayer extrusion.

(iii) A means to obtain an ion exchange membrane precursor film byoverlaying on the fluorinated polymer (B′) layer side of the laminatefilm, a reinforcing fabric and a single layer film in this order, andheat pressing them.

(iv) A step of applying a paste having inorganic particles dispersed ina dispersion medium (such as an aqueous solution of methyl cellulose) toa transfer substrate to form an inorganic particle layer, which is thentransferred on the layer (S′) side of the ion exchange membraneprecursor film, to form a second inorganic particle layer 16.

Step (b):

The ion exchange membrane precursor film is immersed in the alkalineaqueous solution, to subject the groups convertible to carboxylic acidtype functional groups and the groups convertible to sulfonic acid typefunctional groups to hydrolysis treatment to convert them to carboxylicacid type functional groups and sulfonic acid type functional groups,and at the same time, at least part of the sacrificial yarns in thereinforcing fabric embedded in the ion exchange membrane precursor filmis dissolved to form a reinforcing material, to obtain a composite film.Here, the composition and temperature of the alkaline aqueous solutionare within the above-mentioned ranges.

Step (c):

The coating liquid (D) comprising inorganic particles (P), a binder anda dispersion medium is applied to the surface of the layer (C) 12 in thecomposite film. Thereafter, the dispersion medium is removed by e.g.heating to form a first inorganic particle layer 10, to obtain an ionexchange membrane 1.

(Other Embodiments of Production Method for Ion Exchange Membrane)

The production method for an ion exchange membrane 1 is not limited tothe above-described embodiment.

For example, it may be a method of conducting step (c) prior to step(b).

Further, in a case where in step (a), the fluorinated polymer (A) andthe fluorinated polymer (B) are employed, step (b) may not be carriedout.

Further, it may be a method in which on the surface of the layer (C) ofthe laminated film of the layer (C) and the layer (S), the firstinorganic particle layer is formed by the coating liquid (D), and then,the second inorganic particle layer is laminated on the surface of thelayer (S).

Further, it may be a method in which the second inorganic particle layeris formed by applying the coating liquid (D) in the same manner as forthe first inorganic particle layer.

In the production method for an ion exchange membrane of the presentinvention, a fluorinated polymer (A′) is used whereby the proportion ofstructural units having carboxylic acid type functional groups in afluorinated polymer (A) obtainable after hydrolysis treatment will bewithin the above-mentioned range, the temperature of the alkalineaqueous solution in the hydrolysis treatment is within theabove-mentioned range, and the concentration of the water-solubleorganic solvent is within the above-mentioned range, whereby it ispossible to efficiently obtain an ion exchange membrane which has highcurrent efficiency, little variation in current efficiency and highalkali resistance.

The present inventors have found that variation in current efficiency atthe time of electrolyzing an alkali chloride, is caused by variation inthe water content in the layer (C).

As the cause for variation in the water content in the layer (C),variation in the temperature or liquid composition of the alkalineaqueous solution in the reaction system at the time of hydrolyzinggroups convertible to carboxylic acid type functional groups in the ionexchange membrane precursor film, is considered to be a cause, andparticularly, variation in the liquid composition is considered to bethe main cause. Such variation is, of course, likely to occur betweenlots, but may occur even within the same lot.

As a result of an intensive study on how to suppress variation in thewater content in the layer (C), the present inventors have found thatwhen the hydrolysis temperature is low, the hydrolysis reaction becomesslow, the polymer is less likely to be swelled, and paths for ions (ionchannels) in the polymer structure tend to be uniformly formed overtime. However, if the hydrolysis temperature is high, the hydrolysisreaction proceeds rapidly, the polymer tends to be excessively swelled,and ion channels in the polymer structure tend to be non-uniformlyformed. For such a reason, when the temperature of the alkaline aqueoussolution Is at most the upper limit, it becomes less susceptible to aninfluence of fluctuation or variation in the composition or temperatureof the alkaline aqueous solution in the reaction system, and it ispossible to produce an ion exchange membrane having little variation incurrent efficiency.

<Production Method for Alkali Chloride Electrolysis Apparatus>

The production method for an alkali chloride electrolysis apparatus ofthe present invention is a method which comprises obtaining an ionexchange membrane by the production method for an ion exchange membraneof the present invention, and mounting the ion exchange membrane in anelectrolytic cell, so as to partition the inside of the electrolyticcell comprising a cathode and an anode into a cathode chamber on thecathode side and an anode chamber on the anode side.

FIG. 2 is a schematic diagram showing an example of the alkali chlorideelectrolysis apparatus of the present invention.

The alkali chloride electrolysis apparatus 100 has an electrolytic cell110 comprising a cathode 112 and anode 114, and an ion exchange membrane1 mounted in the electrolytic cell 110 so as to partition the inside ofthe electrolytic cell 110 into a cathode chamber 116 on the cathode 112side and an anode chamber 118 on the anode 114 side.

The Ion exchange membrane 1 is mounted in the electrolytic cell 110 sothat the layer (C) 12 will be on the cathode 112 side, and the layer (S)14 will be on the anode 114 side.

The cathode 112 may be placed in contact with the ion exchange membrane1, or may be placed with a space from the ion exchange membrane 1.

The material constituting the cathode chamber 116 is preferably amaterial which is resistant to sodium hydroxide and hydrogen. As such amaterial, stainless steel, nickel, etc. may be mentioned.

The material constituting the anode chamber 118 is preferably a materialwhich is resistant to sodium chloride and chlorine. As such a material,titanium may be mentioned.

(Production Method for Aqueous Sodium Hydroxide Solution)

For example, in the case of producing an aqueous sodium hydroxidesolution by electrolyzing an aqueous sodium chloride solution, anaqueous sodium chloride solution 119 is supplied to the anode chamber118 of the alkali chloride electrolysis apparatus 100; water or anaqueous sodium hydroxide solution 121 is supplied to the cathode chamber116, and the aqueous sodium chloride solution is electrolyzed whilekeeping the concentration of the aqueous sodium hydroxide solution 122discharged from the cathode chamber 116 at a predetermined concentration(e.g. 32 mass %).

The concentration of an aqueous sodium chloride solution 120 dischargedfrom the anode chamber 118 is preferably from 150 to 200 g/L.

The concentration of the aqueous sodium hydroxide solution 122discharged from the cathode chamber 116 is preferably from 20 to 40 mass%.

The temperature in the electrolytic cell 110 is preferably from 50 to120° C.

The current density is preferably from 1 to 6 kA/m².

(Other Embodiments of Alkali Chloride Electrolysis Apparatus)

The alkali chloride electrolysis apparatus in the present invention maybe one provided with an ion exchange membrane obtained by the productionmethod for an ion exchange membrane of the present invention, as adiaphragm, and constructions other than the diaphragm may be known ones.

The electrolytic cell may be a monopolar type wherein a cathode chamberand an anode chamber are alternately arranged with the ion exchangemembrane interposed, so that the cathode chambers one another and theanode chambers one another are electrically in parallel, or a bipolartype wherein the back of a cathode chamber and the back of an anodechamber are electrically connected, so that the respective chambers areelectrically in series.

Since the production method for an alkali chloride electrolysisapparatus of the present invention comprises obtaining an ion exchangemembrane by the production method for an ion exchange membrane of thepresent invention, and then, mounting the ion exchange membrane as adiaphragm in an electrolytic cell, it is possible to efficiently producean alkaline chloride electrolysis apparatus which has high currentefficiency, little variation in current efficiency and high alkaliresistance.

EXAMPLES

Now, the present invention will be described in detail with reference toExamples, but the present invention is not limited thereto.

Ex. 1 to 8 are Examples of the present invention, and Ex. 9 to 17 areComparative Examples.

(Measurement of the Proportion of Structural Units Having FunctionalGroups in Each Polymer)

About 0.5 g of a fluorinated polymer (A′) or fluorinated polymer (B′)was melted and molded into a film form by flat plate pressing, and thisfilm sample was analyzed by a transmission infrared spectrometer,whereby using the respective peak heights of CF₂ peak, CF₃ peak and OHpeak of the obtained spectrum, the proportion of structural units havinggroups convertible to carboxylic acid type functional groups in thefluorinated polymer (A′) or having groups convertible to sulfonic acidtype functional groups in the fluorinated polymer (B′), was calculated.This calculated value was adopted as the proportion of structural unitshaving carboxylic acid type functional groups in the fluorinated polymer(A) or having sulfonic acid type functional groups in the fluorinatedpolymer (B), obtainable after hydrolysis treatment.

(Alkali Chloride Electrolysis Apparatus)

Used as an electrolytic cell (effective current area: 25 cm²) was onewherein an inlet of feed water to a cathode chamber was disposed at alower portion of the cathode chamber; an outlet of the aqueous sodiumhydroxide solution to be formed, was disposed at an upper portion of thecathode chamber; an inlet of an aqueous sodium chloride solution to ananode chamber was disposed at a lower portion of the anode chamber, andan outlet of the aqueous sodium chloride solution diluted by thereaction was disposed at the top of the anode chamber.

As the anode, a punched metal made of titanium (short diameter: 4 mm,long diameter: 8 mm) coated with a solid solution of ruthenium oxide,iridium oxide and titanium oxide, was used.

As the cathode, a punched metal of SUS304 (short diameter: 5 mm, longdiameter: 10 mm) having ruthenium-containing Raney nickelelectrodeposited thereon, was used.

(Current Efficiency and Measurement of Variation in Current Efficiency)

The ion exchange membrane was mounted, so as to partition the inside ofthe electrolytic cell into a cathode chamber on the cathode side and ananode chamber on the anode side, and the first inorganic particle layerof the ion exchange membrane faced the cathode, and the second inorganicparticle layer of the ion exchange membrane faced the anode.

Electrolysis was conducted for one week under conditions of atemperature of 90° C. and a current density of 6 kA/m², while keepingthe cathode side in a pressurized state so as to contact the anode andthe ion exchange membrane, supplying 290 g/L of an aqueous sodiumchloride solution and water, respectively, to the anode chamber and thecathode chamber, maintaining the sodium chloride concentrationdischarged from the anode chamber to be 200 g/L, and maintaining thesodium hydroxide concentration discharged from the cathode chamber to be32 mass %, and the current efficiency after one week was measured.Further, for variation in current efficiency, the current efficienciesof five ion exchange membranes produced under the same conditions, weremeasured, and twice the value of their standard deviation was adopted asthe value of variation.

(Measurement of Alkali Resistance)

In an electrolytic cell, the ion exchange membrane was mounted, so as topartition the inside of the electrolytic cell into a cathode chamber onthe cathode side, and an anode chamber on the anode side, and the firstinorganic particle layer of the ion exchange membrane faced to thecathode and the second inorganic particle layer of the ion exchangemembrane faced the anode.

Electrolysis was conducted for at least 3 days under conditions of atemperature of 90° C. and a current density of 6 kA/m², while keepingthe cathode side in a pressurized state so as to contact the anode andthe ion exchange membrane, supplying 290 g/L of an aqueous sodiumchloride solution and water, respectively, to the anode chamber and thecathode chamber, maintaining the sodium chloride concentrationdischarged from the anode chamber to be 200 g/L, and maintaining thesodium hydroxide concentration discharged from the cathode chamber to be32 mass %, and the current efficiency was measured.

Except that the concentration of sodium hydroxide discharged from thecathode chamber was changed to 4 points of the concentration of sodiumhydroxide by increasing the concentration from 32 mass % to 40 mass % by2% intervals, electrolysis was conducted in the same manner as the abovemethod at each concentration of sodium hydroxide, and the currentefficiency was measured at each concentration of sodium hydroxide.

A graph was prepared by taking the current efficiency on the ordinateand the sodium hydroxide concentration on the abscissa, and plotting therespective points of the sodium hydroxide concentration and the currentefficiency measured as described above. Two points i.e. a point wherethe current efficiency exceeds 94% and is closest to 94%, and a pointwhere the current efficiency is less than 94% and is closest to 94%,were connected by a straight line, and on the straight line, the sodiumhydroxide concentration when the current efficiency is 94%, wascalculated, and the value of the sodium hydroxide concentration wastaken as the concentration for alkali resistance. In a case where thecurrent efficiency at any point was 94%, that point was taken as theconcentration for alkali resistance.

(Fluorinated Polymer (A′1))

CF₂═CF₂ and CF₂═CFOCF₂CF₂CF₂COOCH₃ were co-polymerized to obtainfluorinated polymers (A′1-1) to (A′1-6). The proportion of structuralunits having carboxylic acid type functional groups in the fluorinatedpolymer (A) obtained after hydrolysis treatment is shown in Table 1.

(Fluorinated Polymer (B′1))

CF₂═CF₂ and CF₂═CFOCF₂CF(CF₃)OCF₂CF₂SO₂F were copolymerized to obtain afluorinated polymer (B′1-1) (the proportion of structural units havingsulfonic acid type functional groups in the fluorinated polymer (B)obtainable after hydrolysis treatment: 15.29 mol %).

CF₂═CF₂ and CF₂═CFOCF₂CF(CF₃)OCF₂CF₂SO₂F were copolymerized to obtain afluorinated polymer (B′1-2) (the proportion of structural units havingsulfonic acid type functional groups in the fluorinated polymer (B)obtainable after hydrolysis treatment: 17.76 mol %).

(Ex. 1)

Step (a):

Using an apparatus equipped with two extruders, a film die forco-extrusion and a take-off machine, a laminated film (1) having a layer(C′) with a thickness of 12 μm consisting of the fluorinated polymer(A′1-1), and a layer of the fluorinated polymer (B′1-1) with a thicknessof 68 μm, laminated, was obtained.

Using a film die for monolayer extrusion, a single layer film (2) of thefluorinated polymer (B′1-1) with a thickness of 30 μm was obtained.

A reinforcing yarn of monofilament obtained by rapidly stretching a PTFEfilm and then slitting it in a thickness of 100 denier and a sacrificialyarn of multifilament of 30 denier obtained by aligning and twisting sixPET filaments of 5 denier, were subjected to plain weaving inalternating sequences of two sacrificial yarns relative to onereinforcing yarn, to obtain a woven fabric for reinforcement (thedensity of reinforcing yarns: 10 yarns/cm, and the density ofsacrificial yarns: 20 yarns/cm).

The single layer film (2), the woven fabric, the laminated film (1) anda release PET film (thickness: 100 μm) were, in this order and so thatthe layer (C′) of the laminated film (1) would be on the release PETfilm side, passed through a pair of rolls, laminated and integrated. Therelease PET film was peeled off, to obtain an ion exchange membraneprecursor film.

A paste comprising 29.0 mass % of zirconium oxide (average secondaryparticle diameter: 1 μm), 1.3 mass % of methyl cellulose, 4.6 mass % ofcyclohexanol, 1.5 mass % of cyclohexane and 63.6 mass % of water, wastransferred by a roll press to the layer (S′) side of the ion exchangemembrane precursor film, to attach a second inorganic particle layer.The attached amount of zirconium oxide at the membrane surface was 20g/m².

Step (b):

The second inorganic particle layer-attached ion exchange membraneprecursor film was immersed in an alkaline aqueous solution of dimethylsulfoxide/potassium hydroxide/water=5.5/30/64.5 (mass ratio) at 75° C.and subjected to hydrolysis treatment to convert groups convertible tocarboxylic acid type functional groups and groups convertible tosulfonic acid type functional groups, respectively, to carboxylic acidtype functional groups (—COOK) and sulfonic acid type functional groups(—SO₃K), followed by drying to obtain an ion exchange compositemembrane.

Step (c):

The fluorinated polymer (B′1-2) was subjected to hydrolysis treatment,followed by acid form conversion treatment to sulfonic acid groups(—SO₃H), and dissolved in ethanol, to prepare a 9.5 mass % ethanolsolution. To the ethanol solution, 10.8 mass % of zirconium oxide(average primary particle diameter: 0.4 μm) was added as inorganicparticles, and the binder ratio was adjusted to 0.25, to obtain acoating liquid (D) wherein the average secondary particle diameter ofzirconium oxide was 1.3 μm.

On the surface of the layer (C) in the composite membrane, the coatingsolution (D) was applied by spraying, and the first inorganic particlelayer was deposited to obtain an ion exchange membrane.

The carboxylic acid type functional groups and the sulfonic acid typefunctional groups in the ion exchange membrane were, respectively,converted from the potassium salt type to the sodium salt type, followedby evaluations of the ion exchange membrane. The results are shown inTable 1.

(Ex. 2 to 17)

An ion exchange membrane of a sodium salt type was obtained in the samemanner as in Ex. 1 except that the type of the fluorinated polymer(A′1), the temperature of the alkaline aqueous solution used in step(b), the concentration of dimethyl sulfoxide and the concentration ofpotassium hydroxide were changed as shown in Table 1. Evaluations of theion exchange membrane were conducted. The results are shown in Table 1.

TABLE 1 Proportion of structural Type of units having carboxylicAlkaline aqueous solution Time for Variation fluorinated acid typefunctional DMSO KOH hydrolysis Current in Alkali polymer groups influorinated Temperature concentration concentration treatment efficiencycurrent resistance Ex. (A′1) polymer (A) [mol %] [° C.] [mass %] [mass%] [hrs] [%] efficiency [mass %] 1 A′1-1 14.29 75 5.5 30 <1 96.6 0.5 >402 A′1-1 14.29 55 5.5 30 <3 96.8 0.4 >40 3 A′1-2 13.43 75 5.5 30 <1 97.10.5 37.4 4 A′1-2 13.43 55 5.5 30 <3 97.3 0.4 37.2 5 A′1-3 13.61 75 10 30<1 97.0 0.4 38.0 6 A′1-3 13.61 55 10 30 <1 97.2 0.3 37.8 7 A′1-4 13.8975 30 15 <1 96.8 0.4 39.2 8 A′1-4 13.89 55 30 15 <1 97.1 0.3 39.0 9A′1-1 14.29 95 5.5 30 <1 96.2 1.1 >40 10 A′1-2 13.43 95 5.5 30 <1 96.81.1 37.6 11 A′1-3 13.61 95 10 30 <1 95.1 — — 12 A′1-4 13.89 95 30 15 <1<93 — — 13 A′1-5 12.80 75 10 30 <1 97.5 0.3 <36 14 A′1-6 15.68 75 10 30<1 <93 — — 15 A′1-1 14.29 70 — 25 >50 — — — 16 A′1-7 14.56 95 5.5 95 <195.5 — >40 17 A′1-4 13.89 30 5.5 30 >20 — — — DMSO: dimethylsulfoxide,KOH: potassium hydroxide

In Ex. 1 to 8 wherein a fluoropolymer (A′) was used whereby theproportion of structural units having carboxylic acid type functionalgroups in the fluorinated polymer (A) obtainable after hydrolysistreatment became to be within a specific range, and the temperature ofthe aqueous alkaline solution in the hydrolysis treatment was within aspecific range, current efficiency was high, variation in currentefficiency was little, and alkali resistance was high, at the time ofelectrolyzing an alkali chloride by using the obtained ion exchangemembrane.

On the other hand, in Ex. 9 and 10 wherein the temperature of theaqueous alkaline solution was at least 80° C., variation in currentefficiency became large at the time of electrolyzing an alkali chlorideby using the obtained ion exchange membrane.

In Ex. 11 and 12 wherein the temperature of the aqueous alkalinesolution was at least 80° C., current efficiency was low at the time ofelectrolyzing an alkali chloride by using the obtained ion exchangemembrane.

In Ex. 13 wherein the proportion of structural units having carboxylicacid type functional groups in the fluorinated polymer (A) obtainableafter hydrolysis treatment, was too low, alkali resistance was low atthe time of electrolyzing an alkali chloride using the obtained ionexchange membrane.

In Ex. 14 and 16 wherein the proportion of structural units havingcarboxylic acid type functional groups in the fluorinated polymer (A)obtainable after hydrolysis treatment, was too high, current efficiencywas low at the time of electrolyzing an alkali chloride by using theobtained ion exchange membrane.

In Ex. 15 wherein the alkaline aqueous solution did not contain awater-soluble organic solvent, it took at least 50 hours for thehydrolysis treatment, and it was not possible to efficiently produce anion exchange membrane.

In Ex. 17 wherein the temperature of the alkaline aqueous solution waslower than 40° C., it took at least 20 hours for the hydrolysistreatment, and it was not possible to efficiently produce an ionexchange membrane.

INDUSTRIAL APPLICABILITY

The ion exchange membrane of the present invention is useful as an ionexchange membrane to be used for electrolysis of an alkali chloride toelectrolyze an aqueous alkali chloride solution to produce an alkalihydroxide and chlorine.

This application is a continuation of PCT Application No.PCT/JP2016/076487, filed on Sep. 8, 2016, which is based upon and claimsthe benefit of priority from Japanese Patent Application No. 2015-176814filed on Sep. 8, 2015. The contents of those applications areincorporated herein by reference in their entireties.

REFERENCE SYMBOLS

1: ion exchange membrane, 10: first inorganic particle layer, 12: layers(C), 14: layer (S), 14 a: first layer (S1), 14 b: second layer (S2), 16:second inorganic particle layer, 18: reinforcing material, 100: alkalichloride electrolysis apparatus, 110: electrolytic cell, 112: cathode,114: anode, 116: cathode chamber, 118: anode chamber, 119: NaCl aqueoussolution, 120: dilute aqueous NaCl solution, 121: H₂O or aqueous NaOHsolution, 122: aqueous NaOH solution.

What is claimed is:
 1. A production method for an ion exchange membranefor alkali chloride electrolysis, which is a method for producing an ionexchange membrane for alkali chloride electrolysis having a layercontaining a fluorinated polymer having carboxylic acid type functionalgroups, by immersing an ion exchange membrane precursor film having aprecursor layer containing a fluorinated polymer having groupsconvertible to carboxylic acid type functional groups, in an alkalineaqueous solution containing an alkali metal hydroxide, a water-solubleorganic solvent and water, and subjecting the groups convertible tocarboxylic acid type functional groups to hydrolysis treatment toconvert them to carboxylic acid type functional groups, and wherein theconcentration of the water-soluble organic solvent is from 1 to 60 mass% in the alkaline aqueous solution as100 mass %, the temperature of thealkaline aqueous solution is from 40° C. -to less than 80° C., and theproportion of structural units having the carboxylic acid typefunctional groups in the fluorinated polymer having the carboxylic acidtype functional groups, is from 13.10 to 14.50 mol %, in all structuralunits as 100 mol % in the fluorinated polymer having the carboxylic acidtype functional groups.
 2. The production method for an ion exchangemembrane for alkali chloride electrolysis according to claim 1, whereinthe concentration of the water-soluble organic solvent is from 5 to 50mass % in the alkaline aqueous solution (100 mass %).
 3. The productionmethod for an ion exchange membrane for alkali chloride electrolysisaccording to claim 1, wherein the concentration of the alkali metalhydroxide is from 1 to 60 mass % in the alkaline aqueous solution as 100mass %.
 4. The production method for an ion exchange membrane for alkalichloride electrolysis according to claim 1, wherein the concentration ofthe alkali metal hydroxide is from 5 to 50 mass % in the alkalineaqueous solution as 100 mass %.
 5. The production method for an ionexchange membrane for alkali chloride electrolysis according to claim 1,wherein the alkali metal hydroxide is sodium hydroxide or potassiumhydroxide.
 6. The production method for an ion exchange membrane foralkali chloride electrolysis according to claim 1, wherein thewater-soluble organic solvent is at least one member selected from thegroup consisting of aprotic water-soluble organic solvents, alcohols andaminoalcohols.
 7. The production method for an ion exchange membrane foralkali chloride electrolysis according to claim 6, wherein thewater-soluble organic solvent is at least one member selected from thegroup consisting of dimethyl sulfoxide, methyl alcohol, ethyl alcohol,propyl alcohol, 1-methoxy-2-propanol, triethanolamine, diethanolamine,isopropanolamine, triisopropanolamine, dimethylaminoethanol anddiethylaminoethanol.
 8. The production method for an ion exchangemembrane for alkali chloride electrolysis according to claim 1, furtherhaving a layer containing a fluorinated polymer having sulfonic acidtype functional groups, wherein the ion exchange membrane precursor filmfurther has a precursor layer containing a fluorinated polymer havinggroups convertible to sulfonic acid type functional groups, the ionexchange membrane precursor film is immersed in the alkaline aqueoussolution, and the groups convertible to carboxylic acid type functionalgroups are subjected to hydrolysis treatment and converted to carboxylicacid type functional groups, and at the same time, the groupsconvertible to sulfonic acid type functional groups are subjected tohydrolysis treatment and converted to sulfonic acid type functionalgroups.
 9. The production method for an ion exchange membrane for alkalichloride electrolysis according to claim 1, wherein the ion exchangemembrane for alkali chloride electrolysis further has a layer comprisinginorganic particles and a binder, on at least one outermost layer.
 10. Aproduction method for an alkali chloride electrolysis apparatus, whichcomprises mounting an ion exchange membrane for alkali chlorideelectrolysis obtained by the production method for an ion exchangemembrane for alkali chloride electrolysis as defined in claim 1, in anelectrolytic cell comprising a cathode and an anode, so as to partitionthe inside of the electrolytic cell into a cathode chamber on thecathode side and an anode chamber on the anode side.